1. Field of the Invention
The present invention relates to an optical filter in a light amount adjusting apparatus used for optical equipment such as a camera or the like, and relates to a method of making the optical filter suitable for a process of making an ND filter whose spectral transmittance characteristic is substantially fixed in a visible light area, and a light amount adjusting apparatus and optical equipment which have the optical filter.
2. Description Of Related Art
A diaphragm apparatus has been hitherto provided in optical equipment such as a digital camera, a video camera or the like to adjust the amount of light incident on an image surface. It is normally general in the diaphragm apparatus to adjust the light amount by using a diaphragm blade.
However, particularly when the diaphragm diameter is excessively small for a high-luminance object, resolution is deteriorated due to refraction. Here, the diaphragm diameter is restricted and the amount of transmitted light is restricted by using a Neutral Density filter (hereinafter abbreviated as ND filter) or the like as a light amount adjusting member, thereby repressing a reduction in image quality.
Specifically, the ND filter as a separate member from a diaphragm blade is attached to a part of the diaphragm blade with adhesive agent. When a luminance of the object is high, the diaphragm is not narrowed down to the position in which the diaphragm diameter is excessively small, and the diaphragm aperture is set to a fixed size. Furthermore, the ND filter is located on the optical axis to thereby restrict the amount of transmitted light (to adjust the light amount).
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. H6(1994)-95208 (Document 1), No. H11(1999)-15042 (Document 2), a filter which has a plurality of a light amount adjusting function, that is, has a density gradient is used as the ND filter, and light amount adjustment operation is carried out by advancing/retracting the filter into/from the image-taking optical path.
Furthermore, there have been proposed various diaphragm apparatus designed so that no ND filter is attached to the diaphragm blade and the ND filter moves independently.
Normally, metal film or dielectric film formed by deposition process or the like, or a member achieved by laminating a plurality of films described above is used for the ND filter as the light amount adjusting member in the diaphragm apparatus as described above. This is because above described materials are excellent in optical characteristics and durability.
Additionally, as another ND filter, there are a filter formed by mixing and kneading a light-absorbing dye or pigment into glass or transparent film forming material such as cellulose acetate, PET (polyethylene terephthalate) or the like, or a film formed by coating light-absorbing dye or pigment on a transparent substrate formed of the above material.
Furthermore, as disclosed in Japanese Patent Application Laid-Open No. H5(1995)-173004 (Document 3), a method for manufacturing an ND filter using silver haloid film is proposed.
On the other hand, as disclosed in Japanese Patent Application Laid-Open No. H11(1999)-14923 (Document 4), an optical filter having a density gradient is used as a light amount adjusting member in a laser beam printer or the like even when the spectral transmittance characteristic is not fixed.
However, the ND filter which is achieved by forming the film through the deposition process has a problem that the manufacturing cost is increased since the scale of the manufacturing apparatus is large and the manufacturing process is complicated, etc.
With respect to the method in which dye or pigment is mixed with a forming material of film or the like and the method in which dye or pigment is coated on a substrate, an ND filter having uniform density can be produced, but it is very difficult to produce an ND filter having a density gradient.
Here, when silver haloid film is used as in the case of the ND filter manufacturing method disclosed in Document 3, it is possible to make an ND filter having a density gradient, however, there is a problem that light scattering occurs due to silver particles remaining in the silver haloid film and thus the optical characteristics of the ND filter is deteriorated.